OpenGl add-on libraries are libraries that work with and depend on OpenGL.
GLU stands for OpenGL Utility Library. It is an add-on library for OpenGL which provides procedures to: help create texture mipmaps, map coordinates between screen and object space, and create quadric surfaces and NURBS.
Open Inventor is many things:
1) An add-on library to OpenGL.
2) Defines a standard data format (the .iv format) for 3D data interchange.
3) A scene graph API.
4) A windowing system, and platform, independent 3D graphics library, like OpenGL.
5) Successor to IRIS Inventor and predecessor to VRML.